A post and core is a dental restoration used to sufficiently build-up tooth structure for future restoration with a crown when there is not enough tooth structure to properly retain the crown due to loss of tooth structure to either decay or fracture. In many cases the dental root is removed leaving an empty root canal in the tooth. Typically a thin rigid post (e.g. metal post) is inserted into the root canal and this post provides retention for a “core” which is a build up of material that replaces the lost tooth structure. The post can be cemented within the root canal and the core, which is an artificial preparation provides retention for the crown or coping replacing the tooth. The term “post and core” is also referred to as “post-and-core” and “inlay core”. Post and core restorations are often characterised as “foundation restorations”.
In a root canal procedure the nerve of the tooth is typically removed by the dentist using a dental drill, a so called endodontic procedure, leaving a bore in the tooth. In many cases a special post can be provided that matches the shape of the drill and after drilling the post can be directly cemented in the bore. However, the tooth root canal may have a non-regular structure and the bore in the tooth after removing the root is often also irregular, but even for the regular shapes the depth of the bore may be unknown. No post can thereby match the bore and a custom post must be provided.
A typical procedure when designing a post and core is that the dentist provides an impression of the prepared tooth with the bore and possibly also adjacent teeth and sends it typically to a dental technician at a dental laboratory. From this impression a dental model, such as a gypsum model, can be poured, and the dental restoration including the post and core can now be build from the dental model. The dental technician typically builds the post and core in wax, and then performs an investment casting, such that the real post and core is manufactured in a suitable material, e.g. a metal alloy.
The article “Fabrication of a custom-made ceramic post and core using CAD-CAM technology” by Awad et al. from J. Prosthet Dent 2007; 98; 161-162 discloses the use of CAD-CAM technology to fabricate a custom-made ceramic post and core, which includes the fabrication of a direct acrylic resin pattern, i.e. wax, to capture the anatomy of the canal, and then the pattern is scanned, milled and sintered. The pattern is fabricated by the manual method of lubricating the canal with either water or saliva, then placing autopolymerizing acrylic resin on the plastic core with a brush and placing the post into the canal. The post is maintained in position for a few seconds, and then quickly removed for determining if the entire anatomy of the canal was recorded. If needed, more acrylic resin is applied to the post, the post is placed back into canal before it completely polymerizes, and then moving the post in and out of the canal until it is passively inserted and removed. Then the post pattern is placed into the canal and the core is added. The post and core pattern is attached to the scanning ring of the CAD system, and then scanning and milling of the patterns is performed. Thus according to the disclosure the scanner of the CAD system is used to scan the shape of a custom-made plastic sample post and core for manufacturing the real ceramic post and core on a milling machine, so called copy milling.
The article “Primary Study of CAD/CAM for Individual Post and Core Restorations” by Gu et al. from 2nd Meeting of IADR Pan Asian Pacific Federation and the 1st Meeting of IADR Asia/Pacific Region, September 2009, discloses that a pair of post and core plaster casts with preparations of different positions and shapes were fabricated. The plaster casts were then scanned by CGI (Capture Geometrical Inside) method to acquire the 3-dimensional data. The 3-dimenisional designs of post and core restorations were then conducted in CAD/CAM software for prosthodontics, and at last the restorations were machined by means of rapid prototyping to convert the 3-dimensional data of CAD to objects of resin. Thus according to the disclosure post and core plaster casts are scanned before designing and manufacturing the real resin post and core.
Thus it remains a problem to provide a more efficient and high quality CAD-CAM procedure for providing post and cores.